Bill Clinton backs Barack Obama at convention

In a 45-minute speech Wednesday night that observers described as his best in 20 years, Clinton had the flag waving delegates at the Democratic national convention in Charlotte, North Carolina, literally hanging on to his every word as he formally nominated Obama for a second-term.

"We are fired up," chanted the crowd in the packed Times Warner Cable Arena as without any announcement Obama walked out on the stage as he finished, hugged Clinton, waved to the crowd, grinned widely and walked offstage.

Amid repeated applause, a grey haired visibly weak Clinton framed the Nov 6 presidential election as a clear choice between a man who will promote united values rather than the winner-takes-all mentality of the rival Republicans.

Former US President Bill Clinton with current President Barack Obama. Photo: AFP

"If you want a winner-take-all, you're-on-your-own society, you should support the Republican ticket," Clinton said.

"If you want a country of shared prosperity and shared responsibility -- a we're-all-in-this-together society -- you should vote for Barack Obama and Joe Biden."

Earlier, entering the arena to "Don't Stop" by Fleetwood Mac, a song used in his presidential campaigns, Clinton took the stage to wild applause, and quickly began laying out his case for Obama.

"I want to nominate a man who is cool on the outside, but who burns for America on the inside," he said as he warmed up on his theme against "this Republican narrative, this alternative universe".

"We Democrats, we think the country works better with a strong middle class, for real opportunity for poor people to work into it," he said. "You see we believe 'We are all in this together' is a far better philosophy than 'You are on your own'."

Aiming at independent voters, he said America is "clearly better off" than four years ago and argued that many of the serious problems ailing the economy were "inherited" from Republicans.

With Obama waiting in the wings, Clinton declared to raucous cheers: "No president - not me, not any of my predecessors - no one could have fully repaired all the damage that he found in just four years."

Obama again picks up the mantle of the Democratic Party Thursday to seek "four more years" to finish what he set out to do with a historic win on a promise of hope and change in 2008.